Method to form a photovoltaic cell comprising a thin lamina

ABSTRACT

A very thin photovoltaic cell is formed by implanting gas ions below the surface of a donor body such as a semiconductor wafer. Ion implantation defines a cleave plane, and a subsequent step exfoliates a thin lamina from the wafer at the cleave plane. A photovoltaic cell, or all or a portion of the base or emitter of a photovoltaic cell, is formed within the lamina. In preferred embodiments, the wafer is affixed to a receiver before the cleaving step. Electrical contact can be formed to both surfaces of the lamina, or to one surface only.

RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 12/026,530,filed Feb. 5, 2008, which is incorporated herewith in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to a method to form a thin semiconductor laminafor use in a photovoltaic cell.

Conventional photovoltaic cells are most commonly formed from siliconwafers. Typically such wafers are sliced from an ingot of silicon.Current technology does not allow wafers of less than about 170 micronsthick to be fabricated into cells economically, and at this thickness asubstantial amount of silicon is wasted in cutting loss, or kerf.Silicon solar cells need not be this thick to be effective orcommercially useful. A large portion of the cost of conventional solarcells is the cost of silicon feedstock.

There is a need, therefore, for a method to form a thinner crystallinesemiconductor photovoltaic cell cheaply and reliably.

SUMMARY OF THE PREFERRED EMBODIMENTS

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Ingeneral, the invention is directed to a thin semiconductor lamina foruse in a photovoltaic cell and methods for making such a cell.

A first aspect of the invention provides for a method for forming aphotovoltaic cell, the method comprising: providing a contiguous,monolithic semiconductor donor body having a first donor thickness; andcleaving a portion of the contiguous, monolithic semiconductor donorbody to form a first lamina of semiconductor material, wherein the firstlamina of semiconductor material has a first lamina thickness, the firstlamina thickness between about 0.2 micron and about 100 microns thick;and fabricating the photovoltaic cell, wherein the first lamina ofsemiconductor material comprises at least a portion of the base or ofthe emitter, or both, of the photovoltaic cell.

Another aspect of the invention provides for a method for fabricating aphotovoltaic cell, the method comprising: implanting hydrogen ions intoa semiconductor donor body through a first surface of the semiconductordonor body, wherein ion implantation defines a cleave plane at a depthbelow the first surface of between about 0.2 micron and about 100microns; cleaving a lamina of semiconductor material from the donor bodyalong the cleave plane; and fabricating the photovoltaic cell, whereinthe lamina comprises at least a portion of the base or of the emitter,or both, of the photovoltaic cell.

Still another aspect of the invention provides for a method forfabricating a photovoltaic cell, the method comprising: affixing a firstsurface of a first contiguous, monolithic semiconductor donor body to areceiver; after the affixing step, cleaving a first lamina ofsemiconductor material from the first donor body, wherein the firstlamina of semiconductor material includes the first surface and remainsaffixed to the receiver, and fabricating the photovoltaic cell, whereinthe first lamina of semiconductor material comprises at least a portionof the base or of the emitter, or both, of the photovoltaic cell.

An embodiment of the invention provides for a method for fabricating aphotovoltaic cell, the method comprising: doping at least portions of afirst surface of a semiconductor wafer; implanting hydrogen ions throughthe first surface; affixing the first surface to a receiver; and afterthe affixing step, cleaving a first semiconductor lamina from thesemiconductor wafer, wherein the first lamina comprises the firstsurface, wherein the first surface is bonded to the receiver, whereincurrent is generated within the first lamina when it is exposed tolight.

Another embodiment of the invention provides for a method for making aphotovoltaic module, the method comprising: affixing a plurality ofsemiconductor wafers to a receiver; and after the affixing step,cleaving a semiconductor lamina from each of the semiconductor wafers,wherein each lamina is bonded to the receiver, wherein the photovoltaicmodule comprises the receiver and the laminae.

Still another aspect of the invention provides for a semiconductorlamina comprising a photovoltaic cell, the semiconductor lamina havingsubstantially parallel first and second surfaces, wherein a thicknessbetween the first and second surfaces is between about 0.2 and about 100microns, wherein wiring contacts the first surface but no wiringcontacts the second surface, and wherein incident light enters thephotovoltaic cell through the second surface.

Another embodiment of the invention provides for a semiconductor laminacomprising a photovoltaic cell, the semiconductor lamina havingsubstantially parallel first and second surfaces, wherein a thicknessbetween the first and second surfaces is between about 0.2 and about 100microns, wherein wiring contacts the first surface but no wiringcontacts the second surface, and wherein incident light enters thephotovoltaic cell through the second surface.

Still another embodiment of the invention provides for a lamina ofsemiconductor material, the lamina of semiconductor material havingsubstantially parallel first and second surfaces, wherein the distancebetween the first and second surfaces is between about 1 micron andabout 100 microns, wherein peak-to-valley surface roughness of the firstsurface or the second surface is greater than about 600 angstroms, andwherein the lamina comprises a photovoltaic cell or a portion of aphotovoltaic cell.

An embodiment of the invention provides for a photovoltaic modulecomprising: a receiver; and a plurality of semiconductor laminae bondedto the receiver, wherein each semiconductor lamina is between about 1and about 100 microns thick, wherein each semiconductor lamina comprisesat least a portion of a base, or of an emitter, of a photovoltaic cell.A related embodiment provides for a photovoltaic module comprising: aplurality of laminae, each lamina having a thickness between about 0.2and about 100 microns, each lamina comprising at least a portion of abase or of an emitter of a photovoltaic cell; and a substrate, whereineach lamina is bonded to the substrate. Still another related embodimentprovides for a photovoltaic module comprising: a plurality of laminae,each lamina having a thickness between about 0.2 and about 100 microns,each lamina comprising at least a portion of a base or of an emitter ofa photovoltaic cell; and a superstrate, wherein each lamina is bonded tothe superstrate.

Another aspect of the invention provides for a method for forming adevice, the method comprising: adhering a first surface of asemiconductor body to a receiver, wherein the receiver is metal orpolymer; and cleaving a lamina from the semiconductor body, wherein thelamina comprises the first surface, the first surface remains adhered tothe receiver, and the lamina is between 1 and 80 microns thick.

Another aspect of the invention provides for a method for formingmultiple laminae, the method comprising: cleaving a first lamina from asemiconductor wafer, wherein the semiconductor wafer has a firstthickness less than about 1000 microns and the first lamina has athickness of about 1 micron or more; and after cleaving the firstlamina, cleaving a second lamina from the semiconductor wafer, whereinthe second lamina has a thickness of about one micron or more, wherein,after cleaving the second lamina, the semiconductor wafer has a secondthickness greater than about 180 microns, and wherein the differencebetween the second thickness and the first thickness is at least thecombined thickness of the first lamina and the second lamina.

Still another embodiment of the invention provides for a photovoltaiccell comprising: a lamina having a thickness between about 0.2 micronand about 100 microns, the lamina comprising at least a portion of abase of the photovoltaic cell, wherein the lamina comprisesmonocrystalline, multicrystalline, or polycrystalline semiconductormaterial; and a first amorphous semiconductor layer comprising at leasta portion of an emitter of the photovoltaic cell.

Another embodiment of the invention provides for a photovoltaic devicecomprising: a semiconductor lamina having a thickness between about 1micron and about 20 microns, wherein the lamina has a first surface anda second surface substantially parallel to the first surface, whereinthe lamina comprises at least a portion of a base of a photovoltaiccell, wherein electrical contact is made to both the first surface andthe second surface of the photovoltaic cell; and a substrate orsuperstrate, wherein the lamina is affixed to the substrate orsuperstrate at the first surface or the second surface.

Another embodiment of the invention provides for a method for forming aphotovoltaic cell, the method comprising: depositing a first layer of afirst material on a first surface of a silicon wafer; implanting one ormore species of gas ions through the first surface to define a cleaveplane; affixing the wafer to a receiver at the first surface; heatingthe wafer to exfoliate a lamina from the wafer along a cleave plane,wherein the lamina comprises the first surface and the lamina remainsaffixed to the receiver; texturing the first surface or the secondsurface of the lamina.

Still another embodiment provides for a photovoltaic cell comprising: acrystalline silicon lamina having thickness between about 1 micron andabout 20 microns, wherein the lamina comprises a base and an emitter ofthe photovoltaic cell, the lamina having a first surface, and a secondsurface substantially parallel to the first surface; a substrate,wherein the lamina is affixed to the substrate at the first surface; ametal layer between the lamina and the substrate; and wiring inelectrical contact with the second surface, wherein incident lightenters the photovoltaic cell at the second surface.

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another.

The preferred aspects and embodiments will now be described withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view depicting a prior art photovoltaiccell.

FIG. 2 is a graph of short-circuit current vs thickness of varioussilicon photovoltaic cells.

FIGS. 3 a and 3 b are cross-sectional views showing stages in formationof a photovoltaic cell according to an embodiment of the presentinvention.

FIGS. 4 a through 4 d are cross-sectional views showing stages information of a photovoltaic cell according to an embodiment of thepresent invention.

FIGS. 5 a-5 c are cross-sectional views showing stages in formation of aphotovoltaic cell according to an embodiment of the present invention.

FIGS. 6 a and 6 b are cross-sectional views showing stages in formationof a photovoltaic cell according to another embodiment of the presentinvention.

FIGS. 7 a-7 c are cross-sectional views showing stages in formation of aphotovoltaic cell according to another embodiment of the presentinvention.

FIGS. 8 a and 8 b are cross-sectional views showing stages in formationof a photovoltaic cell according to yet another embodiment of thepresent invention.

FIGS. 9 a-9 d are cross-sectional views showing stages in formation of aphotovoltaic cell according to an embodiment of the present invention.

FIGS. 10 a and 10 b are cross-sectional views showing stages information of a photovoltaic cell according to still another embodimentof the present invention.

FIGS. 11 a and 11 b are cross-sectional views showing stages information of a photovoltaic cell according to still another embodimentof the present invention.

FIGS. 12 and 13 are cross-sectional views of alternative embodiments inwhich a lamina formed according to the present invention is a portion ofa tandem or multijunction photovoltaic cell.

FIG. 14 is a plan view of a photovoltaic module comprising a pluralityof thin photovoltaic cells according to an embodiment of the presentinvention.

FIGS. 15 a-15 c are cross-sectional views showing stages in formation ofan alternative embodiment of the present invention in which a lamina istransferred between a substrate and a superstrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical silicon wafer used to make a photovoltaic cell is about 200 to250 microns thick. It is known to slice silicon wafers as thin as about180 microns, but such wafers are fragile and prone to breakage.

A conventional prior art photovoltaic cell includes a p-n diode; anexample is shown in FIG. 1. A depletion zone forms at the p-n junction,creating an electric field. Incident photons will knock electrons fromthe conduction band to the valence band, creating electron-hole pairs.Within the electric field at the p-n junction, electrons tend to migratetoward the n region of the diode, while holes migrate toward the pregion, resulting in current. This current can be called thephotocurrent. Typically the dopant concentration of one region will behigher than that of the other, so the junction is either a p-/n+junction (as shown in FIG. 1) or a p+/n− junction. The more lightlydoped region is known as the base of the photovoltaic cell, while themore heavily doped region is known as the emitter. Most carriers aregenerated within the base, and it is typically the thickest portion ofthe cell. The base and emitter together form the active region of thecell.

Within certain ranges, conversion efficiency of a photovoltaic cellvaries with its thickness. In this discussion, conversion efficiencyrefers to the fraction of incident photon current that is converted tousable electrical current. As the thickness of a cell is reduced, morelight will pass through it without being absorbed. Additional thicknessallows for more absorption and higher cell efficiency. Light absorptioncan also be improved by increasing the distance light travels throughthe cell by bending it at an oblique angle or by internally reflectingit multiple times through the cell. Bending can be caused, for example,by texturing one or both surfaces of the cell, and reflection by coatingone surface with a reflective material. These effects are known as lighttrapping.

A surface that is textured such that the angles of transmitted andreflected light are fully randomized is called a Lambertian surface.That is, for a Lambertian surface, the photon flux density per unitsolid angle is independent of the direction of the incident light andposition along the surface.

As noted, photovoltaic cells are generally at least 200 microns thick,but need not be. FIG. 2 is a graph showing theoretical short-circuitcurrent density (J_(SC)) vs. thickness for various photovoltaic cells.(FIG. 2 is taken from Green, M. A., (1995) “Silicon Solar Cells,Advanced Principles and Practice,” Centre for Photovoltaic Devices andSystems, University of New South Wales.) It will be seen that for a cellhaving Lambertian surfaces, J_(SC) decreases with decreasing thickness,but relatively gradually. For example, for the curve labeled“Lambertian” in FIG. 2, at about 100 microns, J_(SC) is about 42 mA/cm²,while at 50 microns, J_(SC) has dropped only slightly, to about 41mA/cm²; at 10 microns, J_(SC) is still well above 35 mA/cm².Substantially thinner photovoltaic cells, at 5, 2, 1, and even afraction of a micron thick, can theoretically be made with commerciallyuseful efficiencies, if they can be made at sufficiently low cost.

In embodiments of the present invention, a very thin semiconductorlamina is cleaved from a semiconductor donor body, for example amonocrystalline or multicrystalline silicon wafer, by means other thanconventional slicing, allowing the lamina to be much thinner. The laminacan be processed to form all or a portion of a photovoltaic cell.

Referring to FIG. 3 a, in a preferred embodiment, one or more species ofgas ions are implanted through a first surface 10 of a wafer 20. Theions are slowed by electronic interactions and by nuclear collisionswith atoms in the lattice. The implanted ions reach a distribution ofdepths, some deeper, some shallower. This distribution will have amaximum concentration at some depth below first surface 10. The implantprocess results in lattice damage, also at a distribution of depths. Thedamage consists of vacant lattice sites created by displacement of thelattice atoms due to collisions with the incoming implanted atoms. Thisdamage also has a depth of maximum concentration, which is slightlyshallower than the depth of the maximum concentration of implanted gasatoms. The implant defines a cleave plane 30 along which a lamina can becleaved from the wafer 20. The depth of cleave plane 30 can be betweenabout 0.2 micron and about 100 microns.

As shown in FIG. 3 b, when the wafer is heated, the implanted gas ionsmigrate to cleave plane 30, forming bubbles or micro-cracks. The bubblesor micro-cracks expand and merge, resulting in separation of lamina 40from donor wafer 20.

The much thinner lamina of the present invention is necessarily morefragile than a relatively thick wafer, and must be handled carefully toavoid breakage. Thus in some embodiments, as shown in FIG. 4 a, firstsurface 10 of wafer 20 is processed first, including, for example,doping with p-type and/or n-type dopants, texturing to increase lighttrapping, growth or deposition of films, etc. After doping, gas ions areimplanted through first surface 10, defining subsurface cleave plane 30.Turning to FIG. 4 b, following definition of cleave plane 30, firstsurface 10 is affixed or adhered to a planar surface 60, which will bereferred to as a receiver. As shown in FIG. 4 c, a subsequent thermalanneal causes lamina 40 to exfoliate along previously defined cleaveplane 30; this anneal may also serve to complete bonding of lamina 40 toreceiver 60.

Cleaving creates second surface 62. Additional processing, such assurface texturing, formation of an antireflective layer, doping,formation of wiring, etc., may be performed to second surface 62.Depending on the embodiment, receiver 60 can serve as either a substrateor a superstrate in the finished device, which may be a photovoltaicmodule. In still other embodiments, lamina 40 may be temporarilytransferred to receiver 60, then transferred to some other substrate orsuperstrate. In some embodiments, electrical contact to lamina 40 ismade only at first surface 10 or at second surface 62, while in otherembodiments, electrical contact is made at both first surface 10 andsecond surface 62.

The result is a lamina 40 having thickness between about 0.2 and about100 microns, preferably between about 1 and about 10 microns; in someembodiments this thickness is between about 1 and about 5 microns.Lamina 40 comprises or is a portion of a solar cell. Lamina 40 has beenprocessed on both sides and is affixed to a substrate or superstrate. Asolar panel or photovoltaic module can be fabricated by affixing aplurality of laminae to the same substrate or superstrate. The pluralityof laminae can be formed in the same steps, further reducing cost.

It should be noted that in the process just described, a contiguous,monolithic semiconductor donor body having a first thickness isprovided. A different process is known in the art by which anepitaxially grown layer of crystalline silicon is first formed on, thenseparated from a porous silicon layer. In one example, a silicon wafermay be subjected to anodic etching, which forms a series of voids at ornear the wafer surface. The voids typically have dimensions of a micronor more. An anneal in hydrogen reconstructs a top surface of siliconhaving a separation layer of voids below it. Silicon is epitaxiallygrown on this reconstructed silicon layer by depositing silicon in aseparate step on a single crystal substrate. The epitaxially grown layeris then separated from the original wafer at the separation layer. Thematerial making up the separated layer was grown, and is not a portionof the original wafer; thus the thickness of the wafer is not reduced bythe thickness of the detached layer, only by the thickness of theseparation layer consisting of voids formed by anodic etching.Immediately before the splitting step, the semiconductor wafer has alayer on it which is epitaxially grown, and includes voids; it is not acontiguous, monolithic donor body.

In contrast, in the present invention, a contiguous, monolithicsemiconductor donor body is provided. In general, the donor body has novoids. The cleaved lamina is a portion of the contiguous, monolithicsemiconductor donor body, not a separate layer which is epitaxiallygrown on the body by depositing silicon in a separate step on a singlecrystal substrate. Thus cleaving the lamina from the donor body reducesthe thickness of the original donor body by at least the thickness ofthe lamina.

EXAMPLE Implant and Exfoliation

An effective way to cleave a thin lamina from a semiconductor donor bodyis by implanting gas ions into the semiconductor donor body to define acleave plane, then to exfoliate the lamina along the cleave plane. Forcompleteness, a detailed example will be provided of how to performimplant and exfoliation. Note exfoliation is one form of cleaving. Itwill be understood that this example is provided for illustration only,and is not intended to be limiting. Many details of this example can bealtered, omitted, or augmented while the result falls within the scopeof the invention.

This description will detail implant into a monocrystalline siliconwafer. It will be understood that many other types of semiconductordonor bodies may be used instead. Referring to FIG. 4 a, one or morespecies of ions is implanted (indicated by arrows) through first surface10 of wafer 20. A variety of gas ions may be used, including hydrogen(H+, H₂+) and helium (He+, He++). In some embodiments, hydrogen ionsalone, or helium ions alone, may be implanted; in alternativeembodiments, hydrogen ions or helium ions are implanted together. Eachimplanted ion will travel some depth below first surface 10. It will beslowed by electronic interactions and nuclear collisions with atoms asit travels through the lattice. The nuclear collisions may lead todisplacement of the lattice atoms creating vacancies or vacant latticesites, which are effectively damage to the lattice.

Some ions will travel farther than others, and after implant, there willbe a distribution of ion depths. Similarly, lattice damage is caused ata distribution of depths, this damage distribution lagging slightlybehind the ion distribution. There will be a maximum concentration ineach distribution. If hydrogen is implanted, the maximum concentrationof damage, which is slightly shallower than the maximum concentration ofhydrogen ions, will generally be the cleave plane. If the implantincludes helium, or some other gas ion, but does not include hydrogen,the maximum concentration of implanted ions will be the cleave plane. Ineither case, the ion implantation step defines the cleave plane, andimplant energy defines the depth of the cleave plane. It will beunderstood that this cleave plane cannot be a perfect plane, and willhave some irregularities. If both hydrogen and helium ions areimplanted, it is preferred for their maximum concentration to occursubstantially at or near the same depth, though they may not be exactlythe same. It is preferred, though not required, that the hydrogenimplant is performed before the helium implant.

In other embodiments, other gas ions may be implanted, including neon,crypton, argon, etc., either alone or in combination with helium, withhydrogen, or with hydrogen and helium; or indeed in any combination.These ions have larger mass, so higher implant energies are required toimplant them to the same depth as a smaller mass ion.

If hydrogen has been implanted, hydrogen atoms passivate danglingsilicon bonds by forming Si—H bonds. Atomic hydrogen will readilypassivate the broken silicon bonds present at vacant lattice sites. Insome cases, multiple hydrogen atoms will bond to adjacent silicon atoms,forming a platelet defect. Platelet defects are more fully described byJohnson et al., “Defects in single-crystal silicon induced byhydrogenation,” Phys. Rev. B 35, pp. 4166-4169 (1987), herebyincorporated by reference. Some hydrogen atoms will not bond withsilicon, and will remain free in the lattice as either atomic ormolecular hydrogen. Implanted helium atoms are inert and will not formbonds, and will thus remain free in the lattice.

As shown in FIG. 4 a, the ion implantation step defines cleave plane 30for a subsequent cleaving step. The depth of cleave plane 30 from firstsurface 10 in turn will determine the thickness of the lamina ultimatelyto be produced. As described earlier, this thickness affects theconversion efficiency of the completed cell. In some embodiments, one ormore thin films may have been deposited or grown on first surface 10before implantation.

The depth of the implanted ions is determined by the energy at which thegas ions are implanted. At higher implant energies, ions travel farther,increasing the depth of the maximum concentration of implanted ions, andthe maximum concentration of damage, and thus the depth of the cleaveplane. The depth of the cleave plane in turn determines the thickness ofthe lamina.

Preferred thicknesses for the lamina are between about 0.2 and about 100microns; thus preferred implant energies for H+ range from between about20 keV and about 10 MeV. Preferred implant energies for He+ ions toachieve these depths also range between about 20 keV and about 10 MeV.

During implant, collisions may occur between the ions being implantedand atoms in the ion implanter. At certain known energies, thesecollisions can cause nuclear reactions, creating gamma radiation, alphaparticles, or x-rays. Depending on ion dose rate and shielding, it maybe preferred to avoid the energies that will cause such reactions. Theamount of radiation and its acceptability are however a function of iondose rate and shielding. The topic is more fully discussed by Saadatmandet al., “Radiation Emission from Ion Implanters when Implanting Hydrogenand Deuterium,” Proceedings of the 1998 International Conference on IonImplantation Technology, pp. 292-295, 1999.

In a typical ion implanter, ions are generated in the ion source bycreating a plasma of some convenient source gas or solid. These ions aresubsequently extracted from the source and mass analyzed to select onlythe desired ion species. There may be ions present in the plasma whichare rejected by the mass analysis. In an alternative type of implanterthere is no mass analysis and hence all of the ion species present inthe source plasma are implanted into the wafer target. In the case of ahydrogen plasma, both H+ and H₂+ ions will likely be present. If theions are not subject to mass analysis, both H+ and H₂+ will beimplanted, creating two distribution peaks at different depths. This isless preferred, as it may render the subsequent exfoliation step moredifficult to control. If hydrogen is implanted without mass analysis, itis advantageous to operate the source in a fashion that will produce apreponderance of either H+ ions or H₂+ ions.

As described, an implant leaves implanted ions at a variety of depths. Ahigher energy implant leaves more ions at depths shallower and deeperthan the depth of the maximum concentration than does a lower energyimplant, resulting in a broader distribution of implanted atoms. Thecleaving process proceeds by diffusion of the gas atoms to the cleaveplane; this broader distribution means that a higher implant dose isrequired for a higher energy implant.

As described by Agarwal et al. in “Efficient production ofsilicon-on-insulator films by co-implantation of He+ with H+”, AmericanInstitute of Physics, vol. 72, num. 9, pp. 1086-1088, March 1998, herebyincorporated by reference, it has been found that by implanting both H+and He+ ions, the required dose for each can be significantly reduced.Decreasing dose decreases time and energy spent on implant, and maysignificantly reduce processing cost.

In some embodiments, it may be preferred to additionally implant a smalldose of boron ions, preferably at a depth substantially coinciding withthe target depth of hydrogen and helium ions. Boron causes hydrogen todiffuse faster, reducing the temperature at which the eventualexfoliation of the lamina can be performed. This effect is described indetail by Tong, U.S. Pat. No. 6,563,133, “Method of epitaxial-like waferbonding at low temperature and bonded structure.”

For clarity, examples of implant dose and energy will be provided. Toform a lamina having a thickness of about 1 micron, implant energy forhydrogen should be about 100 keV; for a lamina of about 2 microns, about200 keV, for a lamina of about 5 microns, about 500 keV, and for alamina of about 10 microns, about 1000 keV. If hydrogen alone isimplanted, the dose for a lamina of about 1 or about 2 microns willrange between about 0.4×10¹⁷ and about 1.0×10¹⁷ ions/cm², while the dosefor a lamina of about 5 or about 10 microns will range between about0.4×10¹⁷ and about 2.0×10¹⁷ ions/cm².

If hydrogen and helium are implanted together, the dose for each isreduced compared to when either is implanted separately. When implantedwith helium, hydrogen dose to form a lamina of about 1 or about 2microns will be between about 0.1×10¹⁷ and about 0.3×10¹⁷ ions/cm²,while to form a lamina of about 5 or about 10 microns hydrogen dose maybe between about 0.1×10¹⁷ and about 0.5×10¹⁷ ions/cm².

When hydrogen and helium are implanted together, to form a lamina havinga thickness of about 1 micron, implant energy for helium should be about50 to about 200 keV; for a lamina of about 2 microns, about 100 to about400 keV; for a lamina of about 5 microns, about 250 to about 1000 keV;and for a lamina of about 10 microns, about 500 keV to about 1000 keV.When implanted with hydrogen, helium dose to form a lamina of about 1 orabout 2 microns may be about 0.1×10¹⁷ to about 0.3×10¹⁷ ions/cm², whileto form a lamina of about 5 or about 10 microns, helium dose may bebetween about 0.1×10¹⁷ and about 0.5×10¹⁷ ions/cm².

It will be understood that these are examples. Energies and doses mayvary, and intermediate energies may be selected to form laminae ofintermediate, lesser, or greater thicknesses.

Once ion implantation has been completed, further processing may beperformed on wafer 20. Elevated temperature will induce exfoliation atcleave plane 30; thus until exfoliation is intended to take place, careshould be taken, for example by limiting temperature and duration ofthermal steps, to avoid inducing exfoliation prematurely. Onceprocessing to first surface 10 has been completed, as shown in FIG. 4 b,wafer 20 can be affixed to receiver 60.

Turning to FIG. 4 c, exfoliation of a lamina 40 is most readily effectedby increasing temperature. As described above, the earlier implant stepleft a distribution of gas ions, and a distribution of lattice damage inthe donor silicon wafer, where the implant defined cleave plane 30. Ifhydrogen was implanted, many hydrogen ions broke silicon bonds duringcollisions with silicon atoms and passivated those bonds, in some casesforming platelet defects, as described earlier. These platelet defectsare on the order of 30 to 100 angstroms wide, less than 200 angstromswide, at room temperature. After implant and before cleaving, the waferis a contiguous, monolithic semiconductor donor body having no voidslarger than the platelet defects. Receiver 60 with affixed wafer 20 issubjected to elevated temperature, for example between about 200 andabout 800 degrees C. Exfoliation proceeds more quickly at highertemperature. In some embodiments, the temperature step to induceexfoliation is performed at between about 200 and about 500 degrees C.,with anneal time on the order of hours at 200 degrees C., and on theorder of seconds at 500 degrees C. As temperature increases, theplatelet defects begin to expand as more and more of the unbonded gasatoms diffuse in all directions, some collecting in the plateletdefects, and forming micro-cracks. Eventually the micro-cracks merge andthe pressure exerted by the expanding gas causes lamina 40 to separateentirely from the donor silicon wafer 20 along cleave plane 30. Thepresence of receiver 60 forces the micro-cracks to expand sideways,forming a continuous split along cleave plane 30, rather than expandingperpendicularly to cleave plane 30 prematurely, which would lead toblistering and flaking at first surface 10.

Note that platelet defects will only form when hydrogen is implanted. Ifhelium or other gas ions are implanted without hydrogen, the implantedatoms will form micro-cracks or bubbles that fill up with gas, thencleave along cleave plane 30.

It will be apparent that relative dimensions, for example thicknesses ofreceiver 60, wafer 20, and lamina 40, cannot practically be shown toscale in figures.

It was mentioned previously that coimplanting boron with hydrogen willcause hydrogen to diffuse faster. For this reason, it is expected thatif wafer 20 is p-doped with boron, a common p-type dopant, exfoliationmay be achieved at a slightly lower temperature than if it is intrinsicor lightly n-doped.

In alternative embodiments, other methods, or a combination of methods,may be used to induce exfoliation of lamina 40. For example, methodsdescribed by Henley et al., U.S. Pat. No. 6,528,391, “Controlledcleavage process and device for patterned films,” hereby incorporated byreference, may be employed.

FIG. 4 d shows the structure inverted, with receiver 60 on the bottom.It will be seen that lamina 40 is created by the cleaving step, and thatlamina 40 comprises first surface 10, and has a second surface 62substantially parallel to first surface 10. As will be described, lamina40 comprises or is, or will be, a portion of a photovoltaic cell. Firstsurface 10 remains affixed to receiver 60. In some embodiments theelevated temperature used to perform the exfoliation will also serve tosimultaneously complete the bonding process between first surface 10 andreceiver 60.

It is known to form silicon-on-insulator films for use in thesemiconductor industry by implanting gas ions into a silicon wafer,bonding the silicon wafer to an oxide wafer, and exfoliating a thin skinof silicon onto the oxide wafer. Semiconductor devices, such astransistors, are then fabricated in the exfoliated silicon skin.

The technique of gas ion implantation into a semiconductor wafer andexfoliation of a thin silicon skin has not been used to formphotovoltaic cells, however, despite the fact that material cost is alarge fraction of the cost of most commercial solar cells, and solarmanufacturers face a worldwide silicon shortage. As noted earlier,conventional wafering techniques are hugely wasteful of silicon.

Ion implantation is widely used in fabrication of semiconductor devices,but has been considered impractical for widespread use in the solarindustry, as keeping processing cost low is generally paramount forsolar manufacturers.

Typical high-dose implants used in the semiconductor industry are in therange of 1×10¹⁴ to 3×10¹⁵ ions/cm² at energies up to about 80 keV.Exfoliating a lamina having a thickness of 1-10 microns, for example,requires implant energy of hundreds of keV, and at relatively highdoses, for example 4×10¹⁶ to 2×10¹⁷ ions/cm². Higher implant dose athigher energy increase the cost of the implant.

The inventors of the present invention have recognized that a lamina of100 microns or less, for example 10 microns or less, can be used to forma photovoltaic cell with acceptable conversion efficiency, even wherethe lamina comprises all or portions of the base and/or emitter, theactive regions of the cell. The implantation of gas ions in theembodiments described herein can be performed today on existingimplanters. The inventors believe that use of a specialized,high-throughput implanter would substantially reduce the cost of thisimplant.

For clarity, several examples of fabrication of a lamina havingthickness between 0.2 and 100 microns, where the lamina comprises, or isa portion of, a photovoltaic cell according to embodiments of thepresent invention, will be provided. For completeness, many materials,conditions, and steps will be described. It will be understood, however,that many of these details can be modified, augmented, or omitted whilethe results fall within the scope of the invention. In theseembodiments, it is described to cleave a semiconductor lamina byimplanting gas ions and exfoliating the lamina. Other methods ofcleaving a lamina from a semiconductor wafer could also be employed inthese embodiments.

EXAMPLE Standard Front-and-Back Contact Cell

The process begins with a donor body 20 of an appropriate semiconductormaterial. An appropriate donor body may be a monocrystalline siliconwafer of any practical thickness, for example from about 300 to about1000 microns thick. In alternative embodiments, the wafer may bethicker; maximum thickness is limited only by practicalities of waferhandling. Alternatively, polycrystalline or multicrystalline silicon maybe used, as may microcrystalline silicon, or wafers or ingots of othersemiconductors materials, including germanium, silicon germanium, orIII-V or II-VI semiconductor compounds such as GaAs, InP, etc. In thiscontext the term multicrystalline typically refers to semiconductormaterial having crystals that are on the order of a millimeter in size,while polycrystalline semiconductor material has smaller grains, on theorder of a thousand angstroms. The grains of microcrystallinesemiconductor material are very small, for example 100 angstroms or so.Microcrystalline silicon, for example, may be fully crystalline or mayinclude these microcrystals in an amorphous matrix. Multicrystalline orpolycrystalline semiconductors are understood to be completely orsubstantially crystalline.

The process of forming monocrystalline silicon generally results incircular wafers, but the donor body can have other shapes as well.Cylindrical monocrystalline ingots are often machined to an octagonalcross section prior to cutting wafers. Multicrystalline wafers are oftensquare. Square wafers have the advantage that, unlike circular orhexagonal wafers, they can be aligned edge-to-edge on a photovoltaicmodule with no unused gaps between them. The diameter or width of thewafer may be any standard or custom size. For simplicity this discussionwill describe the use of a monocrystalline silicon wafer as thesemiconductor donor body, but it will be understood that donor bodies ofother types and materials can be used.

Referring to FIG. 5 a, wafer 20 is formed of monocrystalline siliconwhich is preferably lightly doped to a first conductivity type. Thepresent example will describe a relatively lightly p-doped wafer 20 butit will be understood that in this and other embodiments the dopanttypes can be reversed. Dopant concentration may be between about 1×10¹⁴and 1×10¹⁸ atoms/cm³; for example between about 3×10¹⁴ and 1×10¹⁵atoms/cm³; for example about 5×10¹⁴ atoms/cm³. Desirable resistivity forp-type silicon may be, for example, between about 133 and about 0.04ohm-cm, preferably about 44 to about 13.5 ohm-cm, for example about 27ohm-cm. For n-type silicon, desirable resistivity may be between about44 and about 0.02 ohm-cm, preferably between about 15 and about 4.6ohm-cm, for example about 9 ohm-cm.

First surface 10 is optionally treated to produce surface roughness, forexample, to produce a Lambertian surface. The ultimate thickness of thelamina limits the achievable roughness. In conventional silicon wafersfor photovoltaic cells, surface roughness, measured peak-to-valley, ison the order of a micron. In embodiments of the present invention, thethickness of the lamina may be between about 0.2 and about 100 microns.Preferred thicknesses include between about 1 and about 80 microns; forexample, between about 1 and about 20 microns or between about 2 andabout 20 microns. Practically, any thickness in the range between about0.2 and about 100 microns is achievable; advantageous thicknesses may bebetween about 1 and about 1.5, 2, 3, 5, 8, 10, 20, or 50 microns.

If the final thickness is about 2 microns, clearly surface roughnesscannot be on the order of microns. For all thicknesses, a lower limit ofsurface roughness would be about 500 angstroms. An upper limit would beabout a quarter of the film thickness. For a lamina 1 micron thick,surface roughness may be between about 600 angstroms and about 2500angstroms. For a lamina having a thickness of about 10 microns, surfaceroughness will be less than about 25000 angstroms, for example betweenabout 600 angstroms and 25000 angstroms. For a lamina having a thicknessof about 20 microns, surface roughness may be between about 600angstroms and 50000 angstroms.

This surface roughness can be produced in a variety of ways which arewell-known in the art. For example, a wet etch such as a KOH etchselectively attacks certain planes of the silicon crystal faster thanothers, producing a series of pyramids on a (100) oriented wafer, wherethe (111) planes are preferentially etched faster. A non-isotropic dryetch may be used to produce texture as well. Any other known methods maybe used. The resulting texture is depicted in FIG. 5 a. Surfaceroughness may be random or may be periodic, as described in “Niggeman etal., “Trapping Light in Organic Plastic Solar Cells with IntegratedDiffraction Gratings,” Proceedings of the 17^(th) European PhotovoltaicSolar Energy Conference, Munich, Germany, 2001.

In some embodiments, diffusion doping may be performed at first surface10. First surface 10 will be more heavily doped in the same conductivitytype as original wafer 20, in this instance p-doped. Doping may beperformed with any conventional p-type donor gas, for example B₂H₆ orBCl₃. In other embodiments, this diffusion doping step can be omitted.

Next ions, preferably hydrogen or a combination of hydrogen and helium,are implanted to define a cleave plane 30, as described earlier. Beforeimplant, it may be preferred to form a thin oxide layer 19, which may beabout 100 angstroms or less, on first surface 10. Oxide layer 19 mayserve to reduce surface damage during the implant. This oxide, generallysilicon dioxide, can be formed by any conventional method. If diffusiondoping is performed before the implant, providing some oxygen duringdiffusion doping will cause silicon dioxide layer 19 to grow.

Note that the plane of maximum distribution of implanted ions, and ofimplant damage, is conformal. Any irregularities at first surface 10will be reproduced in cleave plane 30. Thus in some embodiments it maybe preferred to roughen surface 10 after the implant step rather thanbefore.

After implant, oxide layer 19 is removed and first surface 10 iscleaned. Once the implant has been performed, exfoliation will occuronce certain conditions, for example elevated temperature, areencountered. It is necessary, then, to keep processing temperature andduration below those which will initiate exfoliation until exfoliationis intended to take place. In general, exfoliation is more readilycontrolled, and the lamina is more easily handled, if the first surface10, through which the implant was performed, is affixed to a receiver ofsome sort to provide mechanical support. In preferred embodiments, tominimize handling, this receiver is in fact a superstrate or substratewhich will be part of the photovoltaic module after fabrication iscomplete. This receiver may be any appropriate material, such assemiconductor, glass, metal, or polymer. Referring to FIG. 5 b, in thepresent example, the receiver to which first surface 10 is affixed is asubstrate 60. In the present embodiment, substrate 60 may beborosilicate glass or some other material that can tolerate relativelyhigh temperature.

A reflective metallic material, for example titanium or aluminum, shouldcontact first surface 10. Other alternatives for such a layer, in thisand other embodiments, include chromium, molybdenum, tantalum,zirconium, vanadium, or tungsten. In some embodiments, it may bepreferred to deposit a thin layer 12 of aluminum onto first surface 10.For example, aluminum can be sputter deposited onto first surface 10.Alternatively, the surface of substrate 60 may be coated with aluminumor some other reflective metallic material. Subsequent thermal stepswill soften the aluminum, causing it to flow and make good contact withfirst surface 10. In other embodiments, an aluminum layer can be formedon both first surface 10 and on substrate 60.

Turning to FIG. 5 c, lamina 40 can now be cleaved from donor wafer 20 atcleave plane 30 as described earlier. Second surface 62 has been createdby exfoliation. In FIG. 5 c, the structure is shown inverted, withsubstrate 60 on the bottom. As has been described, some surfaceroughness is desirable to increase light trapping within lamina 40 andimprove conversion efficiency of the photovoltaic cell. The exfoliationprocess itself creates some surface roughness at second surface 62. Insome embodiments, this roughness may alone be sufficient. In otherembodiments, surface roughness of second surface 62 may be modified orincreased by some other known process, such as a wet or dry etch, as mayhave been used to roughen first surface 10. If metal 12 is a p-typeacceptor such as aluminum, annealing at this point or later may serve toform or additionally dope p-doped region 16 by causing metal atoms frommetal layer 12 to diffuse into region 16.

Next a region 14 at the top of lamina 40 is doped through second surface62 to a conductivity type opposite the conductivity type of the originalwafer 20. In this example, original wafer 20 was lightly p-doped, sodoped region 14 will be n-type. This doping may be performed by anyconventional means. In preferred embodiments this doping step isperformed by diffusion doping using any appropriate donor gas that willprovide an n-type dopant, for example POCl₃.

Diffusion doping is typically performed at relatively high temperature,for example between about 700 and about 900 degrees C., although lowertemperature methods, such as plasma enhanced diffusion doping, can beperformed instead. This elevated temperature will cause some aluminumfrom aluminum layer 12 to diffuse in at first surface 10. This elevatedtemperature can serve as the anneal mentioned earlier to form a moreheavily doped p-type region 16 which will serve to form a goodelectrical contact to aluminum layer 12. If doping of p-region 16 fromaluminum layer 12 is sufficient, the earlier diffusion doping stepperformed at first surface 10 to form this region can be omitted. Ifoxygen is present during the n-type diffusion doping step, a thin layerof silicon dioxide (not shown) will form at second surface 62.

Antireflective layer 64 is preferably formed, for example by depositionor growth, on second surface 62. Incident light enters lamina 40 throughsecond surface 62; thus this layer should be transparent. In someembodiments antireflective layer 64 is silicon nitride, which has arefractive index of about 1.5 to 3.0; its thickness would be, forexample, between about 500 and 2000 angstroms, for example about 650angstroms.

Next wiring 57 is formed on layer 64. In some embodiments, this wiringis formed by screen printing conductive paste in the pattern of wiring,which is then fired at high temperature, for example between about 700and about 900 degrees C. For example, if layer 64 is silicon nitride, itis known to screen print wiring using screen print paste containingsilver. During firing, some of the silver diffuses through the siliconnitride, effectively forming a via through the insulating siliconnitride 64, making electrical contact to n-doped silicon region 14.Contact can be made to the silver remaining above antireflective layer64.

FIG. 5 c shows a completed photovoltaic cell according to one embodimentof the present invention. Lamina 40 is bonded to substrate 60 at firstsurface 10. Incident light enters lamina 40 at second surface 62. Notethat the lightly p-doped body of lamina 40 is the base of this cell,while heavily doped n-region 14 is the emitter; thus lamina 40 comprisesa photovoltaic cell. Current is generated within lamina 40 when it isexposed to light. Electrical contact is made to both first surface 10and second surface 62 of this cell. Wiring 57 is in electrical contactwith second surface 62.

EXAMPLE Front and Back Contact, Photolithographic Wiring

It may be preferred to form wiring 57 by other methods. Referring toFIG. 6 a, fabrication of this embodiment is the same as for the priorembodiment up to the point at which silicon nitride layer 64 has beenformed on second surface 62. At this point a series of parallel trenches68 are formed in silicon nitride layer 64, exposing the silicon ofsecond surface 62 in each trench 68. Trenches 68 can be formed by anyappropriate method, for example by photolithographic masking andetching. Optionally, a second diffusion doping step with an n-typedopant can be performed at this point, more heavily doping siliconexposed in trenches 68.

FIG. 6 b shows wiring 57, which is formed contacting n-doped region 14exposed in trenches 57. Wiring 57 can be formed by any convention means.It may be preferred to form a metal layer on silicon nitride layer 64,then form wiring 57 by photolithographic masking and etching. In analternate embodiment, wiring 57 is formed by screen printing, forexample to form aluminum wiring.

EXAMPLE Localized Rear Contact

In another embodiment, electrical contact at the back surface of thecell is made locally. Referring to FIG. 7 a, this embodiment begins withlightly p-doped wafer 20, which is optionally roughened at first surface10 as in earlier embodiments. Dielectric layer 55, which will act as adiffusion barrier, is deposited on first surface 10. In some embodimentsdielectric layer 55 is silicon nitride or SiO₂, and may be between about1000 and about 1200 angstroms. Vias 68 are formed in silicon nitridelayer 55, exposing first surface 10 in each via 68. Note that inpreferred embodiments, vias 68 are vias, not trenches. A diffusiondoping step is performed, doping exposed areas of first surface 10 witha p-type dopant and forming heavily doped p-type regions 16. In someembodiments this diffusion doping step may be omitted. Next gas ions areimplanted as before, defining cleave plane 30.

As shown in FIG. 7 b, aluminum layer 11 is formed on silicon nitridelayer 55, filling the vias and contacting heavily doped p-type regions16. In some embodiments aluminum layer 11 may be about 1 micron thick.Next wafer 20 is affixed to substrate 60 at first surface 10.

Turning to FIG. 7 c, which shows the structure inverted with substrate60 at the bottom, fabrication continues as in previous embodiments.Lamina 40 is formed by exfoliation from wafer 20, creating secondsurface 62. Second surface 62 may be roughened, as in prior embodiments.An n-doped region 14 is formed by diffusion doping at second surface 62.Elevated temperature during this diffusion doping step causes somealuminum from aluminum layer 11 to diffuse into lamina 40 where itcontacts silicon at first surface 11, further doping p-doped regions 16.Antireflective layer 64 is formed on second surface 62. As in priorembodiments, a thin oxide layer (not shown) may have grown on secondsurface 62 during the diffusion doping step to form n-doped layer 14.Wiring 57 is formed, by screen printing, photolithography, or by someother method, completing the cell.

EXAMPLE Amorphous Emitter and Base Contacts

In another embodiment, the heavily doped regions of the cell are formedin amorphous semiconductor layers. Turning to FIG. 8 a, to form thiscell, in one embodiment, original wafer 20 is lightly n-doped (asalways, in alternate embodiments, conductivity types can be reversed.)First surface 10 of wafer 20 is optionally roughened as in priorembodiments. After cleaning first surface 10, a layer 72 of intrinsic(undoped) amorphous silicon is deposited on first surface 10, followedby a layer 74 of n-doped amorphous silicon by any suitable method, forexample by plasma enhanced chemical vapor deposition (PECVD). Thecombined thickness of amorphous layers 72 and 74 may be between about1000 and about 5000 angstroms, for example about 3000 angstroms. In oneembodiment, intrinsic layer 72 is about 1000 angstroms thick, whilen-type amorphous layer 74 is about 2000 angstroms thick. Gas ions areimplanted through layers 74, 72 and into first surface 10 to form cleaveplane 30 as in prior embodiments. It will be understood that the implantenergy must be adjusted to compensate for the added thickness ofamorphous layers 74 and 72.

A reflective, conductive metal 11 is formed on n-doped layer 74, onsubstrate 60, or both, as in prior embodiments, and wafer 20 is affixedto substrate 60 at first surface 10, with intrinsic layer 72, n-dopedlayer 74, and metal layer 11 intervening between them. Metal layer 11can be aluminum, titanium, or any other suitable material. To facilitateeventual electrical connection to each cell, if metal layer 11 wasdeposited onto substrate 60, it may have been deposited in a pattern,such that the areas to which individual wafers are to be affixed areisolated from each other. These areas of metal 11 may extend for a shortdistance outside of the wafer area, so that electrical contact can bemade to them. This patterning can be done, for example, by depositingthrough a shadow mask; or by etching metal 11 after it is deposited, forexample, through a physical mask placed on substrate 60.

FIG. 8 b shows the structure inverted, with substrate 60 at the bottom.Lamina 40 is exfoliated from wafer 20 along cleave plane 30, creatingsecond surface 62. Second surface 62 is optionally roughened, and iscleaned. Intrinsic amorphous silicon layer 76 is deposited on secondsurface 62, followed by p-doped amorphous silicon layer 78. Thethicknesses of intrinsic amorphous layer 76 and p-doped amorphous layer78 may be about the same as intrinsic amorphous layer 72 and n-dopedamorphous layer 74, respectively, or may be different. Nextantireflective layer 64, which may be, for example, silicon nitride, isformed on p-type amorphous layer 78 by any suitable method. Inalternative embodiments, antireflective layer 64 may be a transparentconductive oxide (TCO). If this layer is a TCO, it may be, for example,of indium tin oxide, tin oxide, titanium oxide, zinc oxide, etc. A TCOwill serve as both a top electrode and an antireflective layer and maybe between about 500 and 1500 angstroms thick, for example, about 900angstroms thick.

Finally wiring 57 is formed on antireflective layer 64. Wiring 57 can beformed by any appropriate method. In a preferred embodiment, wiring 57is formed by screen printing.

In this embodiment, lamina 40 is the base, or a portion of the base, ofa photovoltaic cell. Heavily doped p-type amorphous layer 78 is theemitter, or a portion of the emitter. Amorphous layer 76 is intrinsic,but in practice, amorphous silicon will include defects that cause it tobehave as if slightly n-type or slightly p-type. If it behaves as ifslightly p-type, then, amorphous layer 76 will function as part of theemitter, while if it behaves as if slightly n-type, it will function aspart of the base.

As will be described, preferably a plurality of these cells is formed atone time onto a single substrate 60. Deposition of p-type amorphouslayer 78 and, if it was a TCO, of antireflective layer 64 onto multiplelaminae affixed to the same substrate 60 in the same deposition stepleaves adjacent laminae electrically connected through these layers.These layers must be electrically separated before formation of wiring57, for example by etching these layers through a physical mask that isplaced on the substrate/lamina assembly, or by ablating the layers awaywith a laser.

To finish a panel, the individual cells should be wired together,typically in a series configuration, in which the N+ electrode of onecell is connected to the P+ electrode of the adjacent cell. This can bedone by patterning wiring 57 during its formation so as to make contactto metal surfaces already patterned on substrate 60, if any.Alternatively, wiring 57 can be connected to metal patterns in substrate60 by individual soldering. If there is no metal patterning in substrate60, a laser can be used to ablate the entire lamina 40 from a smallarea, for example about a square cm, of each lamina 40, exposing themetal underneath. This exposed metal can be connected to wiring 57 ofthe neighboring lamina by soldering, for example.

EXAMPLE Back-Contact Cell

Turning to FIG. 9 a, another embodiment begins with lightly doped wafer20 of either type; this example will describe initial wafer 20 aslightly p-doped, but it will be understood that either conductivity typecan be used. First surface 10 is optionally roughened, and doped with afirst conductivity type dopant, for example p-type, forming p-dopedregion 16. Doping may be performed by diffusion doping. A diffusionbarrier 32 is deposited on first surface 10; diffusion barrier 32 may besilicon nitride. Turning to FIG. 9 b, areas of silicon nitride layer 32are removed, exposing portions of first surface 10. A second doping stepis performed, counterdoping the exposed areas of first surface 10 to asecond conductivity type opposite the first, for example n-type, formingn-doped regions 14, which are depicted in cross-hatching. Preferablyboth n-doped regions 14 and p-doped regions 16 are doped to aconcentration of at least 10¹⁸ atoms/cm³.

Turning to FIG. 9 c, next silicon nitride layer 32 is removed, and ionsare implanted to define a cleave plane 30. A dielectric layer 18, forexample silicon dioxide, is deposited or grown on first surface 10. Viasare etched in dielectric layer 18, and wiring is formed on dielectriclayer 18. Wiring is formed in two electrically isolated sets; one wiringset 57 contacts n-doped regions 14, while another wiring set 58 contactsp-doped regions 16. Wiring sets 57 and 58 may be formed by depositing ametal and patterning it photolithographically. A dielectric 22 such asspin-on glass fills gaps between wiring sets 57 and 58 and makes arelatively planar surface. This surface is affixed to substrate 60.Exfoliation is cleaner and more controllable when the surface is planarand uniformly affixed to the receiver, in this case substrate 60.

FIG. 9 d shows the structure inverted with substrate 60 on the bottom.Lamina 40 is cleaved from wafer 20 along cleave plane 30, forming secondsurface 62. Second surface 62 is preferably roughened by any knownmethod. In some embodiments, second surface 62 is doped to the sameconductivity type as that of initial wafer 20. In this example, initialwafer 20 was n-type; thus this surface may be doped with an n-typedopant by diffusion doping to form n-doped region 17. It may bepreferred to flow some oxygen during this diffusion doping step, whichwill cause a thin silicon dioxide layer (not shown) to form; this thinsilicon dioxide layer will help passivate dangling bonds at secondsurface 62, reducing recombination.

Next antireflective layer 64 is formed; antireflective layer 64 may besilicon nitride. Silicon nitride deposited by PECVD will include somehydrogen, and this hydrogen will tend to passivate these dangling bondsat second surface 62, decreasing recombination. Deposition conditionsmay be chosen to increase the hydrogen content of silicon nitride layer64 to increase the amount of hydrogen for this purpose.

In this embodiment, electrical contact in the form of wiring sets 57 and58 is made only to first surface 10. The p-n diode junction is formedbetween heavily doped p-regions 16 and the lightly n-doped body oflamina 40. Photocurrent flows between n-doped regions 14 and p-dopedregions 16. Thus no electrical contact need be made to second surface62. In this embodiment, the base of the photovoltaic cell is the lightlyn-doped body of lamina 40, while the emitter is the combined heavilydoped p-type regions 16; thus lamina 40 comprises both the base andemitter of a photovoltaic cell. As in all of the embodiments describedin this section, current is generated within lamina 40 when it isexposed to light.

EXAMPLE Exfoliation to Superstrate with TCO

In the embodiments so far described, the lamina is exfoliated to asubstrate, where the first surface, the original surface of the donorbody, is the back surface of the finished cell, and the second surfacecreated by exfoliation is the surface where light enters the cell. Thelamina may instead be exfoliated to a superstrate, where the originalsurface of the donor body is the surface where light enters the cell,while the second surface, created by exfoliation, is the back surface ofthe finished cell. Two examples will be provided, though many others canbe imagined.

Turning to FIG. 10 a, in this example semiconductor donor body 20 is alightly p-doped silicon wafer. First surface 10 of wafer 20 isoptionally textured as in prior embodiments. Next a doping step, forexample by diffusion doping, forms n-doped region 14. If oxygen ispresent during this doping step, a thin oxide (not shown) will grow atfirst surface 10. It will be understood that, as in all embodiments,conductivity types can be reversed. Gas ions are implanted through firstsurface 10 to define cleave plane 30.

First surface 10 is cleaned, removing any oxide formed during diffusiondoping. In the present example, TCO 80 will intervene between firstsurface 10 and superstrate 60. This TCO 80 is indium tin oxide, titaniumoxide, zinc oxide, or any other appropriate material, and can bedeposited on first surface 10, on superstrate 60, or both. As TCO 80serves as both a contact and as an antireflective coating, its thicknessshould be between about 500 and about 1500 angstroms thick, for exampleabout 900 angstroms thick. Wafer 20 is affixed to superstrate 60 atfirst surface 10. Note superstrate 60 is a transparent material such asglass.

Turning to FIG. 10 b, lamina 40 is exfoliated from wafer 20 at cleaveplane 30, creating second surface 62. Second surface 62 is optionallytextured. Conductive layer 11 is deposited on second surface 62.Conductive layer 11 is preferably a metal, for example aluminum. Ifconductive layer 11 is aluminum, an anneal forms p-doped layer 16. Ifsome other material is used for conductive layer 11, p-doped layer 16must be formed by a diffusion doping step before conductive layer 11 isformed.

Aluminum layer 11 can be formed by many methods, for example bysputtering with a shadow mask. If the method of formation of aluminumlayer 11 leaves adjacent cells electrically connected, interveningaluminum must be removed to electrically isolate them.

FIG. 10 b shows the completed cell with superstrate 60 at the top, asduring operation. Incident light falls on superstrate 60 and enters thecell at first surface 62.

EXAMPLE Exfoliation to Superstrate with Wiring

As in the prior superstrate embodiment, in FIG. 11 a, lightly p-dopedwafer 20 is optionally textured at first surface 10, then doped to formn-type region 14. Implantation of gas ions at first surface 10 formscleave plane 30.

In this example, antireflective layer 64, for example silicon nitride,is formed on first surface 10, for example by PECVD. Trenches are formedin silicon nitride layer 64, for example by photolithography or by laserscribing, to expose first surface 10. Wiring 57 is formed contactingn-doped region 14. Wiring can be formed of any appropriate conductivematerial, for example aluminum, by any appropriate method, for examplephotolithographic masking and etching.

Next dielectric 22 such as spin-on glass fills gaps between wiring 57and makes a relatively planar surface. This surface is affixed tosuperstrate 60. Superstrate 60 is transparent.

Turning to FIG. 11 b, lamina 40 is exfoliated from wafer 20 at cleaveplane 30, creating second surface 62. At this point fabrication proceedsas in the prior embodiment: Second surface 62 is optionally textured. Insome embodiments a diffusion doping step is performed, forming p-dopedregion 16, while in other embodiments this step can be omitted.Conductive layer 11, preferably aluminum, is formed on second surface 62and an anneal will form p-doped region 16. FIG. 11 b shows the completedcell with superstrate 60 at the top, as it will be during celloperation.

EXAMPLE Multijunction Cells

In alternative embodiments, a lamina formed according to the presentinvention may serve as a portion of a tandem or multijunction cell. Asshown in FIG. 12, the substrate 60 to which lamina 40 is affixed mayalready include a photovoltaic cell or portion of a cell 90; incidentlight will fall first on lamina 40, then pass through it to cell 90.Alternatively, as shown in FIG. 13, another cell or portion of a cell 92may be formed above lamina 40, such that incident light travels firstthrough cell 92, then through lamina 40. In other embodiments, there maybe one or more cells or semiconductor layers above and/or below lamina40. The other cells can be formed of the same semiconductor material aslamina 40 or of a different semiconductor material; examples includegermanium, silicon germanium, GaAs, CdTe, InN, etc. Lamina 40 caninclude at least a portion of the base, or of the emitter, of aphotovoltaic cell, or both.

Lamina 40 and additional cells or semiconductor layers in a tandem ormultijunction cell may be the same semiconductor material, but may havea different degree or grade of crystallinity. For example, lamina 40 maybe monocrystalline silicon while an additional cell or semiconductorlayer is polycrystalline, multicrystalline, microcrystalline, oramorphous silicon, or vice versa. The additional cells forming thetandem or multijunction cell can be formed in a variety of ways, forexample by deposition, by evaporation, by epitaxial growth, by cleavingadditional laminae according to the present invention, or by any othersuitable methods.

A variety of embodiments has been provided for clarity and completeness.Clearly it is impractical to list all embodiments. Other embodiments ofthe invention will be apparent to one of ordinary skill in the art wheninformed by the present specification.

After formation of a first lamina, the semiconductor donor body may besubjected a second time to the implant and exfoliation processes justdescribed to form a second lamina. This second lamina may similarly beaffixed to a receiver and comprise, or be a portion of, a photovoltaiccell, or may be used for a different purpose. As can be imagined,depending on the thickness of the laminae and the thickness of theoriginal donor body, many laminae may be formed, until the donor body istoo thin to handle safely. It may be preferred to form one or morelaminae, then resell the donor body for another purpose. For example, ifa donor body is a monocrystalline silicon wafer having a startingthickness of 400 microns, laminae may be formed by the methods describeduntil the thickness of the donor wafer has been reduced to, for example,about 350 microns. For many applications, there is no practicaldifference between a 400-micron-thick wafer and one that is 350 micronthick; thus the wafer may be resold with little or no loss of commercialvalue.

For example, given a semiconductor donor wafer less than about 1000microns, one, two, three, four, or more laminae can be cleaved from it.Each lamina may have the thicknesses described, for example 20 micronsor less. When the final cleaving step has been performed, the donorwafer is preferably at least 180 microns thick. A wafer at least 180microns thick can still be used for other commercial purposes.

It will be appreciated that because one or more laminae can be formedfrom a wafer without substantially decreasing the value of the donorwafer, the material cost is dramatically reduced. Using the methods ofthe present invention, materials that have previously been consideredimpractical for use in a photovoltaic cell now become economicallyviable. Float-zone silicon wafers, for example, are very high-qualitywafers which have been heat-treated to remove impurities. Typicallyfloat-zone silicon is too expensive for economical use in siliconphotovoltaic cells. Using methods of the present invention, however,laminae of float-zone silicon can be cheaply produced, improving theefficiency of the resulting photovoltaic cell. After cleaving one ormore laminae from the float-zone silicon wafer, the wafer can be resoldand used for another purpose. Other higher-cost source materials, suchas wafers of semiconductor materials besides silicon, for examplemonocrystalline GaAs or mono- or multicrystalline germanium wafers, mayalso be advantageous.

Turning to FIG. 14, a photovoltaic module including multiple laminaeformed by methods according to the present invention may be fabricated.A plurality of donor bodies such as silicon wafers can be processed asdescribed, implanted with gas ions, bonded or otherwise affixed to asingle receiver 88 which is a substrate or a superstrate, and a lamina40 cleaved from each donor wafer in a single cleaving step. The modulecomprises the receiver 88 and the laminae 40. Such a module may includea plurality of laminae 40, for example two, twelve, or more, for examplebetween 36 and 72, or more, or any other suitable number. Each lamina 40comprises or is a portion of a photovoltaic cell, for example at least aportion of its base or emitter. In preferred embodiments, thephotovoltaic cells on the module are electrically connected; they may beconnected in series, as is well known in the art.

In other embodiments of the present invention, it may be preferred totransfer the lamina between two or more receivers to process each side.For example, referring to FIG. 15 a, wafer 20 could be affixed at itsfirst surface 10 to a temporary, high-temperature-tolerant receiver 61.As shown in FIG. 15 b, after cleaving produces lamina 40 having a secondsurface 62, the second surface 62 could be exposed to high temperatureprocesses, for example diffusion doping, without damaging the temporaryreceiver 61. Turning to FIG. 15 c, when processing is complete, lamina40 could be removed from temporary receiver 61 and transferred to afinal receiver 60. It is anticipated that multiple transfers would addcost and reduce yield, however, so these embodiments, while within thescope of the invention, are generally less preferred.

Formation of a lamina affixed to a semiconductor, glass, metal, orpolymer receiver has been described in which the lamina comprises or isa portion of a photovoltaic cell. In alternative embodiments, the laminamay or may not be formed of a semiconductor material, and may be usedfor different purposes. The methods of the present invention may beuseful in any circumstance in which a thin lamina of material is to beaffixed to a receiver; the receiver may be semiconductor, metal,polymer, or some non-insulating material. For example, a semiconductorlamina of one conductivity type or dopant concentration may be affixedto a semiconductor receiver, or a receiver having a semiconductor layer,doped to a different conductivity type or a different dopantconcentration.

Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention.

The foregoing detailed description has described only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration, and not by way oflimitation. It is only the following claims, including all equivalents,which are intended to define the scope of this invention.

1. A method for forming a device, the method comprising: adhering afirst surface of a semiconductor body to a receiver, wherein thereceiver is metal or polymer; and cleaving a lamina from thesemiconductor body, wherein the lamina comprises the first surface, thefirst surface remains adhered to the receiver, and the lamina is between1 and 80 microns thick.
 2. The method of claim 1 wherein the lamina isbetween 1 and 20 microns thick.
 3. The method of claim 1 wherein thesemiconductor body is a silicon wafer.
 4. The method of claim 1 whereinthe lamina comprises a second surface opposite the first, wherein ametal or transparent conductive oxide makes electrical contact to thesecond surface.
 5. The method of claim 4 further comprising, before theadhering step, doping the first surface to a first conductivity type. 6.The method of claim 4 further comprising doping the second surface to asecond conductivity type opposite the first conductivity type.
 7. Themethod of claim 4 wherein a transparent conductive oxide makeselectrical contact to the second surface, and wherein the transparentconductive oxide is zinc oxide.
 8. The method of claim 1 furthercomprising, before the adhering step, implanting one or more species ofgas ions through the first surface, wherein a depth of maximumconcentration of the implanted gas ions defines a cleave plane for thecleaving step.
 9. The method of claim 8 wherein the one or more speciesof gas ions comprise hydrogen ions and helium ions.
 10. The method ofclaim 1 wherein the lamina is between about 1 and about 5 microns thick.11. The method of claim 1 wherein the lamina is between about 1 andabout 2 microns thick.
 12. The method of claim 1 wherein thesemiconductor body is a monocrystalline silicon wafer.
 13. The method ofclaim 1 wherein the semiconductor body comprises polycrystalline ormulticrystalline semiconductor material.
 14. The method of claim 1further comprising, after the cleaving step, fabricating a photovoltaiccell, wherein the photovoltaic cell comprises the lamina.
 15. The methodof claim 1 wherein the receiver is aluminum.